Optical switches play a fundamental role in optical systems and in optical communications systems in particular. The function of optical switches in optical communications is to connect and disconnect transmission paths to rout light beams modulated with information. In other systems optical switches can be used to pulse a light source, e.g., a laser or to perform other functions with modulated or unmodulated light beams. Because optical signals propagate at the speed of light it is important that the optical switches have high switching rates such that they not impede the intrinsically high speed operation of optical systems.
Mechanical optical switches are known. For example, an electro-optically switched directional coupler is proposed by M. Papuchon et al. in "Electrically Switched Directional Coupler: Cobra", Applied Physics Letters, Vol. 27, No. 5, Sep. 1, 1975, pp. 289-291. Further modifications to this switch and similar devices are found in numerous subsequent publications. Although these types of mechanical optical switches are capable of relatively high switching rates, they suffer from many limitations. These limitations include high insertion losses, high susceptibility to temperature variations and other detrimental effects due to external factors. Prior art solutions to the high insertion loss have resulted in devices which are low speed.
In optical communications systems optical switches have to be able to switch the light path of a light beam between optical fibers. Hence, fast switching rates have to be supplemented by high switching precision and low insertion losses to achieve efficient in-coupling into the fibers. Because of these additional requirements the early mechanical optical switches are not suitable for optical communications systems.
Mechanical optical switches adapted to optical communications systems are known. For example, in U.S. Pat. No. 4,239,330 Ashkin et al. teach a multiple optical switch built of two quarter-period graded refractive index lenses sharing a common lens axis. An input fiber delivers a light beam to one of the lenses at a radial offset from common lens axis. A number of output fibers are positioned at the same radial offset and at certain angular displacements on the second lens about the common lens axis. Rotating the lenses relative to each other results in in-coupling of light from the input fiber to different output fibers. The disadvantages of this approach are high insertion loss, low switching speed, low level of reliability and difficulties in making this type of switch.
Another mechanical optical switching device for use with optical fibers is taught by Aoyama in U.S. Pat. No. 4,239,331. This switch utilizes at least one movable transparent dielectric plate positioned between an input fiber and output fibers. The output fibers have associated lenses for in-coupling the light into them. The plate, when placed in the optical path of the light changes its optical transmission path by shifting or offsetting the optical axis of the light from one output fiber to another. U.S. Pat. No. 4,322,126 to Minowa et al. presents a similar mechanical optical switching device which can take advantage of additional prism elements to alter the light path. In a similar vein, U.S. Pat. No. 4,303,303 to Aoyama discloses a variation of the mechanical optical switching device using a parallelogram prism and additional triangular prisms. Unfortunately, the use of additional optical prism elements increases the size of the switching device and introduces a number of additional reflective surfaces in the light path which lead to alignment problems and increased insertion losses.
In U.S. Pat. No. 4,634,239 Buhrer teach a multiple optical fiber electromechanical switch utilizing a rhombic prism. The prism exchanges the optical paths of two parallel beams by means of four refractions and at least two internal reflections. This exchange operation is performed by shifting the beams. The prism's rhombic geometry minimizes the size of the prism and the shift distance to the prism's activated position.
In U.S. Pat. No. 5,361,315 Lewis et al. teach a refractive element optical transmission switch with a fixed position concave reflector and an array of optical input and output waveguides. Rotation of the refractive element is used to couple light from one of the input waveguides to one of the output waveguides.
In fact, none of the prior art mechanical switches are suitable for fast and precise switching between optical fibers. The solutions relying on reflectors are very sensitive to angular variations, while the prism-based solutions are sensitive to variations in shift or offset. Thus, small mis-alignments, thermal effects, mechanical vibration as well as other typical perturbations make it very difficult for those devices to couple light between fibers rapidly while maintaining low insertion losses. The light emitted from the core of the input fiber or waveguide has to be redirected and in-coupled into the core of the output fiber or waveguide. In optical fibers, and especially in single-mode optical fibers, the acceptance cone and the area on which the in-coupling beam has to be focused are small. The low tolerances of in-coupling angle or offset found in the prior art devices limits their usefulness in these applications. What is desired is a device which is relatively insensitive to variations in beam shift and beam angle. Such device should be capable of fast switching rates and use few optical elements in the path between the input fiber and the output fibers.